Critical cooling in crucible free drawing process to produce low melting materials of highest purity



Nov. 17, 1964 H. JACOB 3,157,537

CRITICAL COOLING IN CRUCIBLE FREE DRAWING PROCESS TO PRODUCE LOW MELTING MATERIALS 0F HIGHEST PURITY Filed Jan. 15, 1965 'INVENTOR HERBERT Jecoa BYE n-rToreNEv United States Patent Office 3,157,537 Patented Nov. 17, 1964 3,157,537 CRITICAL COGLING m CRUCHBLE FREE DRAW- ING PROCESS T fliiDUCE W P/EE'LTING MATERIALS 0F HEGEEST PURATY Herbert Jacob, Burghausen, Upper Bavaria, Germany,

assignor to Wacker-Chemie G.m.h.H., Munich, Germany, a corporation of Germany Filed Jan. 15, 1963, 891'. No. 251,507 Claims priority, application Germany Jan. 24, 1962 2 Claims. (Cl. 148-15) This invention relates to the manufacture of high purity chemical compounds, particularly those wth semiconductor properties, and it has for its object to provide a novel and improved process for this purpose.

Another object of the invention is to provide a simple and efiicient process for zone drawing extremely pure rod-shaped semiconductor materials which melt below 1,000 C.

Still another object is to provide a process of the above type in which the melting zone of the rod-shaped material which is being drawn is effectively rinsed and cooled, with the result that the low melting material quickly solidifies and is thus prevented from running or dripping off.

Various other objects and advantages will be apparent as the nature of the invention is more fully disclosed.

It is known to manufacture highly pure compounds with semiconductor properties, such as the antimonides of gallium and indium, by what is known as zone drawing, without using a crucible. In so doing, a rod-shaped object of semiconductor material is supported vertically, as by brackets, inside a quartz tube. The said quartz tube is filled with an inert gas to prevent undesirable side reactions and, if necessary, it is washed continuously. A melting zone, usually created by a high frequency induction heating coil outside the tube, is carried along the rod.

It is also known how to support the melting zone and how to cool it by directing a lateral gas blast against it, and how to influence the solidifying process by cooling devices installed underneath the melting zone and in close proximity thereto.

However, in the case of semiconductor materials melting below 1,000 C. the melting zone at the lower end of the rod does not solidify fast enough to permit an appropriate drawing speed to be maintained. This increases the danger of the melting zone dripping off the rod, particularly in the case of specifically heavy materials with low surface tension.

The present invention provides an improved process for zone drawing low melting materials of the highest purity with a melting point below 1,000 C., in which the said highly pure material in the shape of a rod is arranged vertically and the lower end of the melting zone is cooled by a jet of gas. The process is characterized by the fact that the stream of gas is cooled to a temperature below 0 C. just before it reaches the melting zone, and it rinses the rod from its lower end. I have found that a temperature range is favorable which lies between the temperature of an ice pack and the temperature of liquid nitrogen (-195.8 C.).

The rinsing can be carried out, for instance, by passing the gas stream through a gas washing bottle immersed in a coolant. However, one can also equip the quartz tube around the lower part of the semiconductor rod with a casing and fill the space in between with a coolant fluid, or one can use the gas stream directly as stemming from the vaporizing condensed gases.

By regulating the speed of the gas stream, or by the choice of proper coolants, the process permits setting the cooling effect at will. If condensed gases are used, these can be introduced directly in solid or liquid state into the lower end of the drawing tube, it being possible to regulate the vaporization speed and thus the cooling effect by appropriate changing of the heat insulation of the lower part of the drawing tube. Preferred gases are those with high heat conductivity, e.g., hydrogen, helium and ammonia.

The invention is described in connection with the accompanying drawing which is a diagrammatic illustration of an apparatus suitable for carrying out the process of the invention.

In the drawing, within a quartz tube 1 with an inside diameter of 15 mm. a gallium antimonide rod 2, which was made as a raw polycrystalline rod by known methods, is held at the bottom by a rotatable shaft end 3 and at the top by a rotatable and vertically movable shaft end 4. The quartz tube 1 is surrounded at its lower third by a casing 5 with an inside diameter of mm. and closed at the bottom, filled with liquid nitrogen. At its ends the quartz tube 1 is mounted vertically and gas-tight in the gas-tight ducts 6 through which the rod holder shafts extend. A stream of hydrogen gas, whose speed can be controlled by the regulator valve 7, enters a nipple 8 leading into the lower end of the drawing tube 1, is cooled by the liquid nitrogen while passing the lower third of the quartz tube 1, then passes along the gallium antimonide rod 2, cools the latter at its lower end and leaves the quartz tube 1 through the top outlet 9.

By means of the high frequency coil 10 which is set by known methods for as narrow a melting zone as possible, a melting zone is carried up through the rod, starting from the lower end of the rod, permitting the rod to be drawn out as the shaft end 4 is moved upwardly. If the stream of hydrogen gas is set for a speed of approximately ml. per minute, the melting zone can be moved upward at a drawing speed of about 2 mm. per minute without the zone dripping off the rod 2.

The invention claimed is:

1. Process for drawing a rod of highly pure semiconductor material whose melting point is below 1,000 C., which comprises mounting said rod vertically within a walled enclosure out of contact with the wall of said enclosure, fo'rming a melting zone about the lower end of said rod and thereby rendering molten the portion of said rod which is in said melting zone, pulling the upper end of the rod vertically upwardly within said enclosure to draw out the molten portion of said rod, moving said melting zone vertically in a direction toward the upper end of said rod while continuing to draw said rodvertically upwardly within said enclosure, and continuously passing a gas into said enclosure about the lower end of said rod and thence upwardly along said rod and then out of said enclosure, said gas being at a temperature below 0 C. immediately before it reaches said melting zone.

2. Process according to claim 1, in which said gas enters said melting zone at a temperature between 0 C. and 195.8 C.

References Cited in the file of this patent UNITED STATES PATENTS OTHER REFERENCES Paar: Zone Refining Allied Techniques, George Newnes Limited, London (1960), p. 107. 

1. PROCESS FOR DRAWING A ROD OF HIGHLYPURE SEMICONDUCTOR MATERIAL WHOSE MELTING POINT IS BELOW 1,000*C., WHICH COMPRISES MOUNTING SAID ROD VERTICALLY WITHIN A WALLED ENCLOSURE OUT OF CONTACT WITH THE WALL OF SAID ENCLOSURE, FORMING A MELTING ZONE ABOUT THE LOWER END OF SAID ROD AND THEREBY RENDERING MOLTEN THE PORTION OF SAID ROD WHICH IS IN SAID MELTING ZONE, PULLING THE UPPER END OF THE ROD VERTICALLY UPWARDLY WITHIN SAID ENCLOSURE TO DRAW OUT THE MOLTEN PORTION OF SAID ROD, MOVING SAID MELTING ZONE VERTICALLY IN A DIRECTION TOWARD THE UPPER END OF SAID ROD WHILE CONTINUING TO DRAW SAID ROD VERTICALLY UPWARDLY WITHIN SAID ENCLOSURE, AND CONTINUOUSLY PASSING A GAS INTO SAID ENDLOSURE ABOUT THE LOWER END OF SAID ROD AND THENCE UPWARDLY ALONG SAID ROD AND THEN OUT OF SAID ENCLOSURE, SAID GAS BEING AT A TEMPERATURE BELOW 0*C. IMMEDIATELY BEFORE IT REACHES SAID MELTING ZONE. 